The present invention relates to structured honeycomb substrates formed of metals and metal alloys, and more particularly to honeycomb structured metal substrates for the support of catalysts and/or for the management of temperatures in chemical reactors and heat exchange columns. Methods for making structured metal catalyst supports and heat exchangers by high temperature direct metal extrusion processes are also provided.
In the chemical and petrochemical industry, the performance of many processes is affected by the control and management of the released or consumed heat of reaction. As a result catalyst beds must be efficiently cooled in case of highly exothermic reactions, or heated in the case of endothermic reactions. Examples of highly exothermic reactions include the selective catalytic oxidation of organic compounds such as oxidation of benzene or n-butane to maleic anhydride, o-xylene to phthalic anhydride, methanol to formaldehyde, ethylene to ethylene oxide, and Fischer-Tropsch synthesis. Highly endothermic reactions include the steam reforming of hydrocarbons to syngas (CO and H2).
Processes such as these are frequently carried out in reactors containing a large number of tubes (multi-tubular reactors), typically of the order of centimeters in diameter, loaded with appropriate catalysts in pellet or other form. Generally, such reactors are supplied from the top with reactant feeds, with or without inert components or reaction moderators, with the heat generated or required by the reaction being supplied or removed through the tube walls to a fluid heat exchange medium maintained in the spaces between the tubes. Water, thermal oil, gases, or molten salts are examples of heat exchange media that can be used.
These reactor designs are targeted at keeping the temperature inside the reactor tubes within predetermined narrow ranges since, for example, at high reaction rates the heat released in exothermic reactions can cause local superheating or thermal runaways that can result in significant reaction selectivity losses (e.g. to CO2 in case of partial oxidations), catalyst deactivation or even the destruction of the reactor equipment.
These problems are aggravated by the physical limitations affecting internal heat transfer performance, e.g., the limited heat transfer coefficients and effective radial thermal conductivities of the catalysts and reactor tubes. Common approaches for dealing with these limitations include adjustments such as the staging and or grading of catalyst activity through dilutions or redistributions of the catalysts, limiting reactant throughput, or operating at high fluid flow rates. All of these methods have distinct practical shortcomings, such as increasing catalyst loading complexity, or imposing throughput limitations that reduce reactor operating efficiency, or incurring large pressure drops that again negatively impact process economics.
Catalyst supports formed from corrugated conductive metal sheets by rolling and welding or brazing processes are known, but these typically have shown thermal transfer properties equal to or worse than conventional random packings of catalyst beads, pellets, saddles or other shapes. Mesh-like supports comprising catalysts integrated into layers of fibers or wires have been proposed to enhance radial heat transfer through reactant stream turbulence, but these require efficient radial fluid transport that increases reactor pressure drop.
The use of monolithic honeycomb catalysts or catalyst supports for highly exothermic reactions such as partial oxidations has been proposed to reduce pressure drop but such supports eliminate radial fluid transport as a means of reactor temperature control. A hybrid approach to this problem for highly exothermic reactions employs assemblies of ceramic honeycomb monolithic catalyst sections alternating with packing segments for that promote effective radial mixing and heat transfer within the process stream, but the poor radial heat transfer characteristics of the honeycomb catalyst sections require that significant space be provided for the heat-exchange-promoting segments, resulting in poor reactor space utilization.
Published European patent application EP 1 110 605 provides illustrations of improved honeycomb catalyst designs intended to improve reactor heat transfer in multitubular reactors. These are honeycomb monoliths with interconnecting walls of metals or other thermally conductive materials that achieve radial heat transfer only via thermal conduction through the honeycomb structure itself. Properly implemented, this concept effectively decouples the heat transfer efficiency of a reactor from the mechanisms of radial fluid heat and mass transfer relied on in prior approaches to reactor temperature control. However, conventional metal honeycombs formed by the shaping and layering of metal sheets are typically tack welded constructions that hinder radial heat transfer due to metal contact discontinuities in their radially layered structures.
Channeled metal structures formed by the direct extrusion of metal feedstock have recently been developed for applications such as heat exchangers in HVAC systems. However, these structures are generally one-dimensional channel arrays that if layered into two-dimensional honeycomb channel arrays would present the same hindrances to radial heat transfer as the do the radially layered structures of the aforementioned European application.
Metal honeycombs formed by the extrusion of plasticized powdered metal batches, disclosed for example in U.S. Pat. No. 4,758,272, generally offer heavier constructions featuring thicker walls and wall intersections than sheet-formed honeycombs. However, these extruded honeycombs tend to retain at least some residual internal porosity that can affect strength and interfere with heat conductivity. Further, the batching, forming, and consolidation processes involved in the manufacture of metal honeycomb structures by powder batch extrusion add to the cost of these structures.
In summary, although the various types of conventional metal honeycomb monoliths have found some application in multitubular and other reactor designs for the management of heat in exothermic and endothermic reactions, there is still a need for improved monolith constructions that would provide better heat transfer performance and durability, and that could be manufactured efficiently at reasonable cost.